1. Field of the Invention
The present invention generally relates to methods and systems for improved monitor and control of lithography processes. Certain embodiments relate to a computer-implemented method for separating errors into correctable and non-correctable errors across a field of a lens of a lithography system. Other embodiments relate to a computer-implemented method for controlling one or more parameters of features within substantially an entire printed area on a product wafer.
2. Description of the Related Art
The performance of a lithography system lens has an impact on the features that are printed by the lithography system. Currently, there are two basic methods used to track lens performance. In the simplest approach, the user measures critical dimensions (CDs) for different feature types at a large number of points across the lens field. The raw CD measurement method is simple and well understood. The metrology results will show if the range of CD values measured across the field are within the specification limits or not. Pass/fail decisions are made based on the range of the measured values or another statistical summary of the results. For example, all of the data may be pooled together to get an overall CD distribution. The lens passes if the width of the distribution is within spec.
No other information on lens quality is collected. The exception is that vertical and horizontal features may be looked at separately, which could indicate an astigmatism problem if there is a marked offset between the two. For example, the CD of horizontal and vertical features, usually at fixed CD and pitch but sometimes at a few different values of CD and pitch, may also be measured. CD measurements may also be performed in a few fields at + and − defocus. Global tilt may be removed from the data, but is inadvisable. The process of lumping all of the data together and computing the range is fairly simple, even if flier rejection algorithms are included.
However, due to the interaction between different feature types and the optical aberrations of the lens system, a great number of measurements may be required on different types of features at different locations in the lens field. In addition, the results are equally simplistic. For example, there is no indication of the spatial pattern of the CD distribution and no way to tell if specific locations within the lens field are behaving anomalously. This approach makes it difficult to detect spatial drifts, i.e., changes in the performance of specific locations in the lens over time, or changes in the spatial signature of the lens over time. More importantly, it also makes it difficult (if not impossible) to tell why the lens performance is changing. For example, if the CDs on the right side of the lens begin to be larger than those on the left, it is not possible to determine if there is a problem in the lens itself or just a tilt of the field which could easily be corrected in the focusing system. If the user just subtracts an average value to try to compensate the tilt, the user could miss seeing a serious problem. Therefore, it is not possible to determine if the cause of any excessive CD variation is due to systematic, correctable terms such as a focus offset or focal plane tilt or if the variation is due to non-correctable terms. This method cannot separate correctable from non-correctable contributions to the CD error.
A more complex method is to determine the Zernike aberrations across the pupil plane of the lens as viewed from any point in the lens field. Examples of methods that may be used to determine lens aberrations are illustrated in U.S. Pat. No. 6,248,486 to Dirksen et al., “Novel aberration monitor for optical lithography,” P. Dirksen et al., Proc. of SPIE Vol. 3679, p. 77-86 (1999), and “Impact of high order aberrations on the performance of the aberration monitor,” P. Dirksen et al., Proc. of SPIE Vol. 4000, pp. 9-17 (2000), which are incorporated by reference as if fully set forth herein. Knowing the Zernike coefficients, the CD of any feature can be simulated based on how the image will be projected through the pupil plane.
The method involving the determination of the Zernike aberration coefficients is more rigorous, but also much more difficult to apply in practice and does not provide a direct measurement of the CD performance on the wafer. CDs can be predicted through simulation, but the results will be imperfect due to other lens errors, especially flare, and the detailed interaction of the light with the photoresist. It is difficult to negotiate pass/fail specifications with suppliers based on Zernike values due to the highly proprietary nature of lens design. If a lens fails a qualification test due to one or more Zernike terms being out of spec, the supplier would still insist on time consuming tests to prove that the lens performance has been degraded beyond its specifications for specific photoresist features. Although a large number of methods to determine Zernike coefficients have been proposed, many of these lack adequate sensitivity to the lowest order terms to determine the focal plane error (Z0) at each point in the lens field.
The lithography process often plays a significant role in the success or failure of semiconductor manufacturing. Therefore, lithography processes are often closely monitored for process control purposes. Some methods for monitoring lithography process performance involve measuring features printed on a wafer. For example, currently, users measure a metrology test target (often a “tuning fork” with an isolated and dense array at a fixed CD) at a limited number of points within the scribe line. In some cases, users may also measure the CD at a very limited number of points within the device die itself. If the metrology measurements are within specified limits (“in spec”), the entire lot of wafers is assumed to be acceptable.
A state of the art integrated circuit will have tens or hundreds of millions of structures, all of which must be within spec in order for the device to function properly. These structures will vary widely across the circuit including isolated and dense structures as well as those of intermediate pitch and with varying target CDs across the device. Some process layers may contain mixtures of lines and spaces or contact holes and linear patterns. Knowing that a limited number of test structures within the scribe line is in spec for a fixed CD and a fixed pitch is not a clear indication that all of the device geometries everywhere within the device are within acceptable limits. In addition, the change in dose or focus which will give the optimal dimensions of the test structures may not be the correct dose/focus combination to optimize the CD distribution of the patterns within the device die.
The lithography exposure tool's optical characteristics and the exact type of features being patterned both have a strong influence on CD performance when patterning advanced design rule integrated circuits. Of particular importance are the optical aberrations of the lithographic lens system as well as the systematic dose and focus errors across the field. Many examples exist today of methods used to quantitatively measure these optical aberrations. Three examples of these, each based on slightly different physical principles, are the Litel reticle concept, the Artemis concept by ASML, and the phase shift grating of Toshiba. In each case, the output of the analysis tool is typically provided in terms of Zernike polynomial coefficients, which can accurately describe the induced phase error across the exit pupil of the lithographic lens, and which can be easily interpreted in optically meaningful terms such as spherical, astigmatic, and coma aberrations. Furthermore, a Zernike polynomial description of the aberrations is required for each field point of interest. Although these aberration descriptors are generally accepted as quantitative metrics for the quality of lens systems, it is a non-trivial problem to quantitatively estimate the impact they will have on CD, or more specifically, on the specific dimensions of different types of features within the integrated circuit. Such calculations require detailed knowledge of other process parameters such as the exposure tool illumination configuration, partial coherence, the geometry of the pattern, and the response of the photoresist process to different aerial image profiles.